When AC motors such as synchronous and induction motors are started, a large current is drawn. This starting current is typically five times larger than the rated current of the motors. If several motors are supplied from a common AC bus, which in turn is supplied from a power converter, the converter's current capability must at least be as big as the sum of the rated currents of all motors which are already running plus the starting current of the motor which is started last. This current capability value assumes that the motors are only started one after the other and not multiple of them at the same time. If more than one AC motors are started simultaneously then the current capability of the converter must of course be even higher.
A common AC bus for several AC motors is for example known from offshore platforms producing oil or gas. Such platforms are equipped with huge compressors and pumps which require supply voltages of several kilovolts. More and more of offshore platforms receive their electrical power supply from shore instead of producing it directly on the platform by using gas turbines or diesel generators. The power from shore is usually supplied by a high voltage direct current (HVDC) transmission system, where alternating voltage is transformed by an onshore power converter into direct voltage, transferred via a subsea DC cable to the platform and transformed into alternating voltage by an offshore power converter. The alternating voltage output from the offshore power converter is supplied to an AC bus, i.e. an AC line with one or more phases. The driving units of compressors and pumps on the platform, i.e. synchronous and induction motors, are connected directly to the AC bus without power transformation or conversion in-between. If these motors are started, the current on the AC bus should not exceed the current capability of the offshore power converter.
In order to be on the safe side it is one possibility to provide a power converter with increased current capability. However, such overrating is undesirable due to higher cost and bulkier and heavier equipment. The aspect of weight plays especially an important role when designing an offshore platform.
A known solution to reduce the starting current of larger three-phase AC motors is the so called wye-start-delta-run method, where the motor windings are wye connected during the start and delta connected afterwards. The wye connection applies a reduced voltage compared to the delta connection. The level of the voltage is always reduced by the same factor (1.732) compared to the line voltage. This means that no correction is possible if the voltage is too small to produce the necessary starting torque for the motor. Apart from that, additional switching means are required and switching currents occur during the change from wye to delta connection.
Another solution which is applied in case of induction motors is the installation of extra devices such as starting capacitors. This would again increase the costs.